Prostheses for the joint of the knee have to be designed in order to observe two main rules:                a tribological rule: they have to undergo minimum wear during use;        a kinematic rule: the operation of the prosthesis should be as comfortable as possible for the patient in all daily activities.        
And above all, tribology should meet kinematics and conversely kinematics should be compliant with tribology. It is within the scope of this dual converse obligation that the present invention is located.
In this field of knee prostheses, many prior proposals of total knee prosthesis have already been made. These total prostheses traditionally include three functional elements: a femoral element intended to be implanted on the lower end of the femur, a tibial element adapted so as to be implanted on the upper end of the tibia, and finally an articular insert, generally in polyethylene intended to be inserted between the femoral element and the tibial element. The articular insert is conformed in order to allow and guide movements of the femoral element with respect to the tibial element in all the movements of the prosthetic joint in flexure and/or rotation according to kinematics as close as possible to those of the anatomical knee.
For this purpose, the femoral element includes two condyles, medial and lateral condyles respectively, delimiting between them a so-called intercondylar notch and the articular insert itself has an upper surface including two glenoid cavities, the surfaces of which are of a shape substantially mating the external surfaces of the condyles of the femoral implant and connected together through a protruding interglenoidal connecting surface intended to be housed in the intercondylar notch.
Moreover, the articular insert also has a lower surface configured in order to co-operate with the tibial element. The latter has a planar supporting plate of the insert in which a substantially cylindrical orifice is formed and the lower surface of the insert, as for it, is planar and includes a pin for blocking and pivoting the insert on the tibial element intended to be inserted into the orifice of the tibial element.
In a way known per se, the compliance or congruence between the contact surfaces of the femoral component and the insert is a determining element allowing reduction of wear of the articular surfaces, notably of the insert. Consequently, as the surfaces are more or less fitted into each other, releasing the movement is accomplished by pivoting the insert on the tibial component around a vertical axis, whether it is central or shifted sideways.
Such knee prostheses are traditionally said to be with a “mobile plate” and the prosthesis of the present invention is preferentially of this type.
Depending on the patients to be implanted, the prosthesis may finally include a patellar component covering the posterior portion of the patella and intended to be jointed with the femoral element in its trochlear portion up to a flexural angle of about 60°, and then partly with the condylar portion beyond this flexural angle.
With the purpose of allowing a more or less physiological movement of the joint, several solutions have been proposed in the state of the art.
A first of these configurations consists of forcing the displacement of the femoral element of the prosthesis on the upper surface of the articular insert via a cam formed by a pin or ramp located between the glenoid cavities of the insert, this cam co-operating with a transverse abutment formed in the intercondylar notch of the femoral element. This type of prostheses is said to be “postero-stabilized”.
This prosthesis configuration does not allow the prosthetic knee to adapt to all the movements since it imposes a constant posterior displacement; further the flexion-extension movements of the joint are movements constrained by the co-operation of the cam and of the abutment, therefore unidirectional and identical movements under all circumstances and which may therefore provide bothersome abutment and repulsion sensations, notably at the patella.
Another configuration consists of producing a knee prosthesis said to be with “guiding surfaces”, i.e. for which only the topography of the contact surfaces of the femoral and tibial elements and of the insert of the prosthesis are involved in solving physiological, tribological and stability requirements of the prosthetic joint.
Such prostheses with guiding surfaces are generally without any planar contact surface, any protruding edges and/or abutment element on the articular surfaces of the femoral element (condyles, intercondylar notch, and femoral trochlea) as well as on the upper surface of the articular insert (glenoid cavities and interglenoidal connecting surface).
Among the latter, document WO 00/23 011 describes a prosthesis including a contact surface between insert and femoral component such that the contact point between femoral component and insert is shifted forwards or backwards in each adjacent sagittal plane. This configuration provides point-like or linear and non-surface contacts between femoral element and insert of the prosthesis, responsible for high pressure points, therefore strong point-like or cutting mechanical stresses on the articular insert of the prosthesis, accelerating wear of the latter.
In other proposals such as patent applications FR 2 852 819, and WO 2005/122 967 the femoral component is laid in a cup at the upper face of the insert, in which it is maintained by a wall formed with the whole of the anterior portion of the insert which prevents any rotary movement between both of these portions, without generating a specific movement.
Finally, other proposals such as the one described in document EP 1 354 371 A1, have avoided any central contact between the dome of the insert and the intercondylar notch of the component which suppresses the central guiding effect of the dome of the insert and amounts to having a bicondylar prosthesis bearing upon both glenoids. Moreover, in this type of prosthetic joint of the knee, the medial and lateral condylar supports in the glenoid cavities of the insert are asymmetrical, creating several sources of problems. One of them lies in the fact that a loosening torque is created by compression in the medial cavity, clearly greater than the compression in the lateral cavity. Consequently, the tibial element tends to loosen on the (external) lateral edge of the knee.
Moreover, most of these prostheses have satisfactory congruence in the areas close to the extension, but this congruence substantially decreases upon flexure because of the gradual reduction of the condylar radii of curvature, in the posterior portion of the joint, while the radius of curvature of the tibial plate remains constant.